By correlation one can get, according to a predetermined rule of evaluation, a measure of how well the contents of the memories coincide at different relative displacements.
Essential for the correlation and hence also for the tracking process is the way in which the other memory, hereinafter referred to as the reference image memory, is updated. In known correlation followers, see e.g. U.S. Pat. No. 3,828,122, updating is brought about in the course of one single scanning cycle, the updating process being repeated either periodically after a certain number of cycles, or when the maximum correlation drops below a preselected value. If an image element irrelevant to the tracking process appears momentarily in the section, the first of these updating alternatives entails the danger of the reference image memory being updated precisely when the image element appears, which may jeopardise the tracking process. With the other updating alternative the appearance of the image element may trigger updating of the reference image memory. This leads in practice to tracking of the irrelevant image element.
The object of the present invention is to bring about such updating of the reference image memory that the above disadvantages are avoided and this is enabled in that the video correlator has means arranged to select at each scanning cycle addresses of positions in the reference image memory for the purpose of updating said memory randomly or according to a predetermined rule of selection so that said memory at each scanning cycle is updated only partly and in positions that are distributed over the area of the memory.